1. FIELD OF THE INVENTION
The present invention relates to an original document detecting device for detecting a size of an original document, and more particularly to an original size detecting device for automatically selecting any one of copying sheet sizes and selecting a suitable magnification and minification rate of copying operation in accordance with the size of the original document for use in a copying apparatus or the like. Also the device can detect the presence or absence of a document.
2. DESCRIPTION OF THE PRIOR ART
There is known a copying machine having an original size detecting device provided therein for detecting a size of an original document disposed on its original placing platform and automatically selecting a magnification and minification rate (referred to as MAG/MIN rate hereinafter) of a copying operation and a suitable copying paper size among various paper sizes, thereby preventing an operator from performing an erroneous copying operation. Various kinds of original size detecting devices have been proposed and among them is a well known a kind of detecting device that has a plurality of pairs of light emitting elements and light receiving elements.
Specifically, there are provided a plurality of pairs of light emitting elements and light receiving elements respectively on both sides of an original placing platform holding an original document thereon so as to constitute a plurality of detecting mechanisms, thereby detecting presence or absence of an original document. Such pairs of detecting mechanisms are respectively provided on the positions of the both sides of the original placing platform corresponding to the sizes of the original documents such as A4 and B5 for example, so that the light emitted from one or more emitting elements is interrupted by the original document and is not received by the corresponding light receiving elements, whereby the size of the original document is detected.
In such a conventional detecting device for detecting whether the original document is present or absent, the intensity of the output light emitted from the light emitting elements is fixed so as to be constant. That is say, in the case of using a light emitting diode as a light emitting element, the value of the voltage applied to the light emitting diode is fixed to be constant all the time.
The output signal of the light receiving element is amplified to a predetermined value by an amplifier means, and upon detecting the amplified output value of the light receiving element, a judgment is made whether or not an original document is placed on the original placing platform. In such a conventional detecting device, there is an influence due to disturbance light which effects a detection signal of a light receiving element and the output signal corresponding to the disturbance light received by the light receiving element is also amplified by the amplifier at the same time, therefore, the S/N ratio becomes small, resulting in occurrence of an erroneous detection of an original size. In particular, in case the intensity of the output light emitted by the light emitting element is fixed to be constant, the influence of the disturbance light may be larger than that of regular light emitted by the light emitting element, resulting in affecting detrimentally the light receiving element. Moreover, the output signal of the light receiving element is not stable due to changes of the light emitting and light receiving elements with the time lapse and changes of the environment such as temperature, moisture or the like, so that it becomes impossible to detect the size of the original document accurately.
Although it is considered that the S/N ratio of the output signals of the light receiving element is raised by raising the amplification rate of the amplifier, since not only the regular output signals of the regular light emitted from the light emitting elements but also the output signals due to other disturbance light are amplified by the amplifier at the same time, the effect for improving the S/N ratio can be little obtained. In particular, the more the disturbance light, the more the noise due to the disturbance light becomes even if the output signals of the light receiving elements are amplified, and therefore, the problems mentioned above can not be completely eliminated.
In this case, if the quantity of the light emitted from the light emitting elements is constant, the output value of the light receiving elements is constant so long as the characteristics of the light receiving elements are not changed. The output signal of the light receiving elements is not constant, however, due to the changes of the photo diodes with the time lapse, changes of the environment of the light emitting elements and light receiving elements or to an influence of disturbance light, so that the presence or absence of the original document is apt to be erroneously detected.
Therefore, when the output signal of the light receiving elements is amplified through an amplifier, the variation value of the output signal of the light receiving elements is revised by changing the amplification rate. However, according to a method of amplifying the output signal of the light receiving elements, since the noise component received by the light receiving element is also amplified, the S/N ratio is not completely improved. In case the amplification rate is set in the condition with high output of the light receiving elements, if the light receiving signal is reduced too small due to a change of the condition of the light receiving elements, the signal of a level necessary for detecting the original document can not be obtained.